The present invention relates to fuel cells arranged in fuel cell stack assemblies that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants. In particular the invention relates to a fuel cell stack having a light, electrically insulating pressure plate and a compact current collector, both of which have a low thermal capacity.
Fuel cells are well-known and are commonly used to produce electrical energy from reducing and oxidizing reactants fluids to power electrical apparatus such as apparatus on-board space vehicles, transportation vehicles, or as on-site generators for buildings. A plurality of planar fuel cell plate components are typically arranged into a cell stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids as part of a fuel cell power plant. Each individual fuel cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy. The fuel cell plate components also frequently include a plurality of coolant plates dispersed between the fuel cells. Coolant fluid typically cycles through the coolant plates to maintain the fuel cells and fuel cell stack at an optimum temperature.
It is well known to utilize thick current collecting plates at opposed ends of the cell stack in electrical contact with end plates of the stack in order to collect electrical current generated by the fuel cells of the stack, and to direct the current through cables from the current collectors to a load to perform work. It is also known to utilize thick metal pressure plates at opposed ends of the cell stack, wherein the pressure plates typically include a fastening assembly such as a plurality of tie rods extending between the pressure plates that serve to apply a compressive force on the current collectors and fuel cell plate components between the pressure plates.
For example, in U.S. Pat. No. 4,728,585 that issued on Mar. 1, 1988, to Briggs, which patent is owned by the assignee of all rights in the present invention and which patent is hereby incorporated herein by reference, a steel pressure plate with an adjacent thin, porous graphite plate impregnated with PTFE are shown replacing a thick collector plate and an overhanging, steel pressure plate. The thin, PTFE impregnated graphite plate provides an electrically conductive gas seal between the steel pressure plate and the end plate of the fuel cell component plates, so that the steel pressure plate may also serve as a current collector. U.S. Pat. No. 5,009,968 that issued on Apr. 23, 1991 to Guthrie et al. and to the assignee of all rights in the present invention also shows a fuel cell end plate structure that includes a membrane that is sufficiently thin that compressive forces applied through the membrane and adjacent cells by opposed pressure pads will maintain the membrane in intimate electrical contact with electrodes at multiple locations.
U.S. Pat. No. 6,001,502 that issued on Dec. 14, 1999 to Walsh, also shows combined structural pressure or end plates and current collectors as xe2x80x9cconductive bodiesxe2x80x9d, wherein select surface areas of the conductive bodies are treated with an electrical conductivity xe2x80x9cisolating materialxe2x80x9d to permit electrical conduction from the fuel cells into the conductive body, and to insulate fastening structures contacting the bodies and fluid headers defined within the bodies.
It is also well known that for steel or aluminum combined pressure plate/current collectors, the plates of stainless steel or aluminum are often plated with a precious metal such as gold at substantial cost. The gold plating serves to minimize contact resistance because both aluminum and stainless steel develop non-conductive oxide surfaces when exposed to air and water or water vapor in a typical PEM fuel cell operating environment. It has been shown, however, that such gold plating is both costly and unreliable due to poor adhesion between the gold and base metal.
Known fuel cell stack pressure plates and current collectors are thus typically bulky, complex devices that are costly to manufacture, and that also contribute a substantial weight and volume penalty to overall fuel cell stack specifications. While fuel cell stacks utilized in stationery power plants may operate acceptably with known pressure plates and current collectors, fuel cell stacks utilized to power transportation vehicles must minimize the weight, volume and cost of the fuel cell stack. Additionally, fuel cell stacks utilized within transportation vehicles must be able to start quickly in sub-freezing conditions. Known, heavy, dense metallic pressure plates and/or current collectors exhibit very high thermal mass or thermal capacities. Therefore, they absorb a great deal of thermal energy during a cold start, effectively extending the start-up period. Such high thermal capacity plates also create a thermal lag during a cold start between the fuel cells adjacent to the pressure plates and those fuel cells within the interior of the cell stack. Such a thermal lag prevents rapid start of the fuel cell, which is undesirable.
Accordingly, there is a need for a fuel cell stack having pressure plates and current collectors that are light, less costly to manufacture, and that have substantially reduced thermal capacities.
The invention is a fuel cell stack having an improved pressure plate and current collector. The fuel cell stack produces electricity from reducing fluid and process oxidant reactant streams, and comprises a plurality of fuel cell component plates stacked adjacent each other to form a reaction portion of the fuel cell stack. The plurality of fuel cell component plates include a first end cell component plate at a first end of the stack of fuel cell component plates, and a second end cell component plate at an opposed second end of the stack of fuel cell component plates. A current collector is secured adjacent to an end cell component plate. In one embodiment, the current collector is made from a non-porous, electrically conductive graphite material. In an alternative embodiment the current collector may consist of a thin conductive metal layer (such as a 2 millimeter (xe2x80x9cmmxe2x80x9d) thick copper layer) secured to an electrically conductive graphite material layer so that the graphite material layer is secured adjacent the end cell component plate. The xe2x80x9celectrically conductive graphite materialxe2x80x9d may consist of a pure graphite or a graphite-polymer composite. The current collector also includes at least one conductive stud secured to the collector and extending away from the current collector in a direction away from the end cell component plate. The fuel cell stack also includes a pressure plate secured adjacent to the current collector, wherein the pressure plate overlies the end cell component plate adjacent to the current collector, and the pressure plate is made of an electrically non-conductive, nonmetallic, fiber reinforced composite material.
In a preferred embodiment, the fuel cell stack also includes a thermal insulator secured between the current collector and the pressure plate, for restricting movement of heat from the current collector to the pressure plate and to the surrounding environment. In an additional preferred embodiment, the conductive stud of the current collector may pass through an opening defined within the pressure plate. The current collector may also include a pair or more of conductive studs passing through the pressure plate. The fuel cell stack may also include a first current collector secured adjacent the first end cell component plate and a first pressure plate secured adjacent the first current collector, while a second current collector and second pressure plate are secured in similar fashion adjacent the second end cell component plate and the first and second current collector are both made from a non-porous, conductive graphite material, and the first and second pressure plates are both made of a nonconductive, non-metallic, fiber reinforced composite material.
Accordingly, it is a general object of the present invention to provide a fuel cell stack having an improved pressure plate and current collector that overcome deficiencies of prior art fuel cell stacks.
It is a more specific object to provide a fuel cell stack having an improved pressure plate and current collector that exhibit less weight, volume, and thermal capacity than known fuel cell stack pressure plates and current collectors.
It is yet another object to provide a fuel cell stack having an improved pressure plate and current collector that may be manufactured with greater facility and at a substantially lower cost than known fuel cell stack pressure plates and current collectors.
It is a further object to provide a fuel cell stack having an improved pressure plate and current collector that virtually eliminate corrosion problems associated with known fuel cell stack pressure plates and current collectors.
It an additional object to provide a fuel cell stack having an improved pressure plate and current collector that is safer than known fuel cell stacks having metallic pressure plates because the improved pressure plate of the present invention includes no electrically conductive material exposed to an outside environment surrounding the fuel cell stack.
It is yet a further object to provide a fuel cell stack having an improved pressure plate and current collector wherein non-conductive pressure plates eliminate a need for electrically isolating tie rods passing through the pressure plates, which tie rods apply a compressive load to the plates, thereby reducing cost, simplifying design of the fuel cell stack, and reducing construction and assembly time of parts of the fuel cell stack.
These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.